SFM Piper cub.40 kit 67 ins wing span.

[paul coleman 1]

Okay cheers

[andy pattinson]

Hi Paul , I have just finished building the SFM J3 Cub and am also experiencing exactly the same problems as your self with the recommended electric set up, On its maiden flight it took off pretty level until throttle was applied and it rolled hard left and ended up in a hedge causing some damage to the left wing, wing mounting, and it managed to pull  the tail off, all of this has been repaired now, but i took it back to the fields after repairs and it almost did the same thing again, since then i have found the following a page which will not allow me to post the link, but if you type in left turning tendencies on your search engine and look for a site called boldmethod.com it explains it in good old plain English.

 

Does you cub plan show any side or down thrust on the motor mount, I have lost my plans and cant seem to get a copy any where, it was just something a club member mentioned the other day, I have the SFM Mustang as well , that has side thrust / down thrust built into the motor mount and that flies really well so far.

 

Edited By andy pattinson on 17/05/2019 09:39:08

Edited By andy pattinson on 17/05/2019 09:41:26

[andy pattinson]

Just found this as well which explains a lot.

THE EFFECTS OF POWER

In the single-engine prop-driven aircraft that many modellers fly, as many as four factors can add up to create a swing on take-off.

The first of these is the slipstream of the prop, which takes the form of a vortex that runs around the fuselage in a helical path and eventually encounters one side of the fin, setting up a yawing moment which causes a tendency to swing.

If you’re a tail-dragger pilot, you’ll also have to contend with the effects of blade asymmetry, which will be most marked when the prop axis is not in line with the flight direction of the aircraft, that is, at the start of the take-off run, when the tail’s down. In this condition, the length of the path of the blades through the air will vary as a result of two factors. One of these is the difference in the angle of attack of the blades: The down-going blade meets the relative airflow at a higher angle of attack than the up-going blade, and so generates more thrust. The other cause is the fact that, in their tilted state, the blades are effectively travelling forward through the air at different speeds. The easiest way to visualise how this comes about is to imagine an aircraft moving forward in a nose-high attitude. In the time taken for a blade starting at the top of the tilted disc (the rearmost position) to rotate to the bottom (the foremost position), it will have travelled further through the air (distance travelled by the aircraft + distance of forward rotation) than a blade starting at the bottom of the disc (the foremost position) and rotating to the top (the rearmost position), which amounts to distance travelled by the aircraft - distance of backward rotation. This gives rise to extra thrust on the long-path half of the disc, and so to more yaw.

In a right-handed engine, then - one in which the top of the prop moves to the right when viewed from the cockpit - the extra thrust is on the down-going right-hand side, yawing the model left. The unfortunate thing, of course, is that blade asymmetry starts doing its thing from the start of the take-off run, when you’re winding on power - increasing slipstream effect, in other words - and when your rudder is at its least effective.

[andy pattinson]

You’d think that picking up the tail would at least put you on a level footing with nose-wheel pilots, but no - at least, not until you’ve dealt with the third consideration - torque gyroscopic effect. The force applied to move the axis of the prop’s rotation - that is, to tilt the aeroplane’s longitudinal axis - will be modified by precession so that it acts at a point displaced by 90° in the direction of rotation. Think of it as a finger pushing the top of the prop disc from behind; in a right-hand engine, the precessed force acts as a push on the right-hand side of the disc, compounding the leftwards yaw.

The fourth and final factor is torque effect. The equal and opposite reaction to the force which rotates the propeller tries to rotate the airframe in the opposite direction. On take-off, this causes one wheel to be pressed more firmly onto the ground than the other, giving rise to more friction on that side and therefore another yawing force. On a right-hand engine, you’ve guessed it, it’s the left wheel that drags, compounding the leftward yaw even more.

For all of these reasons, then, you can see why on the take-off run, the throttle should be advanced at a rate such that the reactions are never greater than the ability of the rudder’s increasing authority to control them.

[Denis Watkins]

Have just seen this post, and this model is my most well behaved Cub, in the air and on the ground.

My favourite, of the two VQ I have, has an SC 52 fourstroke fitted, and the ground run is not long enough to go one way or the other

But is easily corrected by breathing on the rudder

The electric set up comes out at 700W, but could fly with less

This one too tracks straight, and takes to the air easily with the massive wing and no nose up

Just allow the wing and groundspeed to lift the model, following previous flying comments by the guys

C of G is crucial on these wide winged, short fuz, huge tailed models, and is balanced level

None of this nose well down for good luck balancing, just get it right

[Frank Skilbeck]

Also bear in mind that electric motors generate instant torque so snapping the throttle open it will generate a lot of torque as it accelerates the prop one way............ and the airframe the other. I had an E Flite Beaver, lovely model but you had to advance the throttle smoothly or it would torque roll on take off. My radio allows a throttle open slow, so I use this too.

[Reno Racer]

I can’t add much more to the excellent advice already provided by others. I tend to build more than a fly at the minute, but have flown lots of tailwheel models, Cubs, decathlon’s, WW1 and tiger moths.

The reason I don’t fly models much these days is because I fly full size, mainly a Citabria ( or Decathlon) which exhibits similar characteristics as a cub, only shorter coupled, aerobatic and more power.

Almost all tailwheel aircraft have their CoG behind the main wheels, which can make take offs pretty tricky, the worst aircraft what to swap ends If you’re not careful - the ground loop/swerve. When I take off in the Citabria, I advance the throttle slowly (about 3 -4 seconds) and then push the stick forward to the neutral position to lift the tail. All the while I need to counteract the torque pulling it to the left with a resting foot of right rudder. I keep it in this position until I’ve reached flying speed ( in this instance 65) before gently lifting off. Once I’m off the ground I immediately feel any effect of crosswind even more; fly relatively flat for a while before slowly climbing into my best rate of climb speed. If there is a cross wind when taking off, then in addition to all the above, when starting the take off run, I also have my aileron into the wind and counter with opposite rudder, whilst remembering to keep a bit of right rudder as well.

Models tends to be the same and needs to be flown in a similar manner, albeit, I find it much harder to do all the above in a model, then a a full size! Yanking an aircraft off too early and not allowing speed to build up when on the main wheels will lead to a stall and the angle between the relative airflow and the able of attack to create a stall, certainly no full size is not great at all and the speed with which it stalls will increase with both weight and angle of bank.

none of the above it tricky in both models or full size, it just takes practice.

Edited By Reno Racer on 18/05/2019 21:09:01

[PatMc]

Posted by Reno Racer on 18/05/2019 21:07:53:

Almost all tailwheel aircraft have their CoG behind the main wheels,

If they didn't they would rest on their nose when stationary.